The overall goals of this research are to identify the neurotransmitters used by subgroups of the retina's neurons, and to learn the specific roles of those neurons in the retina's function. The first aim is to learn the shape and function of a subset of retinal neurons defined by their release of acetylcholine. The cholinergic neurons of the rabbit retina appear to be amacrine cells; they bracket the inner plexiform layer symmetrically, about half of them located in the conventional position of amacrine cells and half "displaced" to the ganglion cell layer. The displaced cholinergic amacrine cells will be studied by intracelular recording and injection of fluorescent or enzymatic markers. The functional activity of the cholinergic cells will be further characterized by measurements of the retina's release of acetylcholine made with fast temporal resolution: acetycholine release will be measured in the dark, during application of Co++, during steady light, and at stimulus onset and cessation. Finally, at neuronal terminals contained within the two main sublaminae of the inner plexiform layer. This will be done by autoradiography of retinas preloaded with radioactive acetylcholine and then stimulated under conditions dominated by either On or Off transients. A second aim is to identify the cells responsible for the apparent presence of the renin-angiotensin system in the retina. Renin, angiotensin I, angiotensin II, and converting enzyme will be assayed biochemically. The cellular location of renin and the angiotensins will be established immunohistochemically. Because renin-containing neurons were found in preliminary experiments only in the ganglion cell layer, biochemical and histochemical analysis will be carried out both in normal retinas and retinas whose optic nerves had been sectioned 6 months previously; immunohistochemistry will be done after marking of the ganglion cell bodies by fluorescent dyes applied to the optic tract; and the renin and angiotensin content of ganglion cells purified by fluorescence-activated cell sorting will be determined.